When it was over – Nanami knelt down, almost collapsing. Whereupon she started coughing and retching aggressively.
Chocho had failed to do her any harm.
Even that final lunge, where she burned through his remaining energy, had been easy enough for her to dodge – in which case, this was more about her constitution.
Nanami Yasuri had no stamina.
And to boot – her body was in awful shape.
You could say she was in tatters –
So pale and fragile, it had been a sight to see her unveil such a fury, but although she had not let Chocho hit her even once, this level of exhaustion – did not befit a martial artist.
The fact is – even if Togame the Schemer had been privy to Nanami being so much stronger than her brother, she would likely have not chosen her to join her on her quest. Nanami could not sustain her strength, which made her poorly suited for the journey.
Nanami’s infirmity was fatal.
Even if she won – she could not win forever.

Katanagatari Omnibus 2, Page 65